The invention relates to a memory element and to a method for fabricating a memory element.
A memory element of this type and a method for fabricating a memory element of this type are known from [1].
In the memory element which is known from [1], organic complexes are provided for the purpose of electrically coupling first lines in a first metallization layer to second lines in a second metallization layer. However, the organic complexes are introduced into the memory element during the individual fabrication processes for the further components of the memory element, in particular prior to the fabrication of at least some of the wiring planes and metallization layers and of the corresponding contacts.
Examples of organic complexes of this type whose electrical conductivity can change by up to a factor of 104 on account of an electric voltage which is applied to the organic complexes, i.e. to the organic material, are known, for example from [2], as N-(3-nitrobenzylidene)-p-phenylenediamine (NBPDA) or as a system of the two materials 3-nitrobenzal-malononitrile (NBMN) and 1,4-phenylenediamine (pDA).
A further organic material which changes its electrical conductivity as a result of an electric voltage applied to the material is known from [3] as rotaxane.
These organic materials are highly sensitive and can easily be damaged, in particular with regard to their behaviour in terms of changing their electrical conductivity.
One drawback of the memory element which is known from [1] is therefore to be seen in particular as residing in the fact that the organic material which is introduced during the method steps, which in some cases are carried out under a great heat, as part of the fabrication method of the memory element, for example during a silicon process, is very easily damaged by the heat.
Therefore, an organic memory element of this type is highly susceptible to faults and its electrical properties are not very robust.
Furthermore, [4] has disclosed a memory cell arrangement and its use as a magnetic RAM memory element and as an associative memory.
Furthermore, [4] has disclosed a drive circuit for the individual memory cells of the magnetic RAM memory element, for writing and reading binary information into and from the respective memory cells via word lines and bit lines.
[5] describes a memory arrangement having a film of organic material.
[6] describes a ROM having a substrate, to which substrate electrodes are applied. A layer is arranged between the electrodes. The ROM consists of conjugated polymers or oligomers and doping atoms.
Further electrically addressable storage media are described in [7], [8], [9], [10], [11], [12] and [13].
Therefore, the invention is based on the problem of describing a memory element with organic materials and a method for fabricating a memory element of this type, which memory element has a reduced susceptibility to faults compared with the memory element which is known from [1].
A memory element has a substrate in which individual transistors and electrical components of the memory element are usually already present. A first metallization layer, preferably of gold, is applied to the substrate, for example a silicon substrate, and a first insulation layer is applied to the first metallization layer. A second metallization layer, which preferably likewise consists of gold, is applied to the first insulation layer, which is preferably made from plasma dioxide or plasma nitride.
The first metallization layer and the second metallization layer are arranged above one another in parallel planes, so that the individual metallization layers do not touch one another. The first metallization layer is patterned in such a manner that first electric lines, which are in each case electrically insulated from one another by, for example, the first insulation layer, are formed therein.
Furthermore, second electric lines, which are arranged above the first electric lines of the first metallization layer in such a manner that they cross one another but, on account of the first insulation layer arranged between them, are not in electrical contact with one another, are formed in the second metallization layer.
At at least some of the crossing points between the first line and a second line, there is in each case one trench, which at least partially overlaps the first line and directly couples, preferably completely overlaps, the second line.
The term crossing point is to be understood as meaning, for example, a point at which, as a result of a substantially perpendicular being raised on one of the two lines, in each case the other line is also touched.
Each trench, or at least some of the trenches, is/are filled with an organic filler material, the electrical conductivity of which can be changed by an applied electric voltage. The change should be sufficiently strong and enduring for it to be easily and robustly possible to record two different states of the organic material, according to whether there is a voltage applied or no voltage applied, in order reliable writing and reading of binary information to and from the respective memory cell is ensured.
In this context, all organic materials whose electrical conductivity may change in particular by up to a factor of 104, preferably by up to a factor of 103 or 102, are suitable.
The filler material may therefore include, for example, organic complexes, preferably the materials
rotaxane, and/or
N-(3-nitrobenzylidene)-p-phenylenediamine (NDPDA), and/or
a system of the materials 3-nitrobenzal-malononitrile (NBMN) and 1,4-phenylenediamine (pDA)
which are known from [2] and [3].
Beneath the two metallization layers, i.e., for example, between the metallization layers and the substrate or even beneath the substrate, there may be a peripheral electronic circuit, which enables the memory element, in particular the individual memory cells which are formed by in each case a filler material with which a trench has been filled, to respond unambiguously and in a highly robust and fault-insensitive way, for example by a binary value being written to or read from a memory cell.
In this memory element, the binary information is given by the corresponding conductivity of the filler material which is arranged in the trenches between the two lines, which represent the word lines and bit lines of the memory element.
The result is a memory element with organic complexes which is unsusceptible to faults and has a very high integration density, since a memory cell has a minimum space requirement of 4*F*F (F=feature size), i.e. in each case the minimum patterning size of the fabrication process used.
If gold is used for the individual lines, one advantage of the memory element is, inter alia, that, by means, for example, of the known gold-sulphur coupling, the corresponding organic materials bond very well to the electric lines, in particular by means of a covalent bond.
A further advantage when using gold for the individual lines is that the surfaces of the individual lines are not oxidized or are only oxidized to a very minor extent.
The memory element has the advantage in particular that the organic complexes only have to be incorporated after the silicon processing, with the result that damage to the organic material under a high temperature load which is required for individual silicon element fabrication steps does not occur.
Since the entire peripheral circuit may be arranged beneath the respective cell field, the chip surface area which remains active, i.e. is completely available for the memory element, is increased further.
The memory element may have additional wiring layers, in order for the individual electronic components provided in the silicon substrate to be electronically coupled to one another.
Furthermore, a second insulation layer may be provided between the substrate and the first metallization layer, in order for the substrate and the first metallization layer, i.e. in particular the first electric lines, to be electrically insulated from one another.
In a method for fabricating a memory element, a first metallization layer is applied to a substrate and is patterned in such a manner that first lines are formed, which are brought into electrical contact with the substrate. This can be achieved, for example, by forming contact holes between the first lines and the substrate, through a second insulation layer provided between them, so that electrical contact can be made between the first lines and the substrate. A first insulation layer is applied to the first metallization layer, and a second metallization layer is applied to the first insulation layer and is patterned in such a manner that second lines are formed, which although they do not electrically couple the first lines do cross them above the first lines. At least at some of the crossing points between the first lines and the second lines, a trench is formed, which in each case partially overlaps the first line and couples the second line, i.e. preferably completely overlaps the second line. The trenches are filled with a filler material, for example an organic filler material as described above, so that the first lines and the second lines can in each case be electrically coupled to one another via the filler material. As explained above, the filler material has a conductivity which changes considerably as a result of an applied electric voltage.